Conventionally, a projection lens of a projector is constituted by many lenses in order to suppress difference (aberration) between an ideal image (an image inputted to the projector) and an actual image (an image projected through the projector). In order to suppress chromatic aberration of magnification, however, many sets of lenses made of materials with different refractive indexes are used, and the number of lenses increases. Therefore, there are problems of increase in apparatus size and increase in manufacturing cost. Furthermore, though the material quality of the lenses is fully considered or aspheric lenses are adopted in order to suppress barrel or pincushion distortion aberration, the design cost and manufacturing cost of the lenses are increased.
Recently, in the case where distortion and the like occur in an image due to chromatic aberration of magnification or distortion aberration caused by a factor in an optical system, a method is generally used in which image processing of an input image is electrically performed as preprocessing of the optical system to give distortion in a direction opposite to the direction of the distortion caused in the optical system, to the input image in advance. For example, in the case where such distortion aberration that an image distorts into a barrel shape occurs in an optical system, correction processing is performed at a stage before the optical system to transform the image into a pincushion shape.
Since random access to an input image is required in correction processing in a conventional projector, a method has been used in which an input image corresponding to one frame is held with the use of a frame memory to access a necessary pixel. However, since a frame memory has a large capacity, there is a problem of increase in manufacture cost and apparatus size. This problem is especially remarkable when input/output of an image is restricted and a double buffer is required.
Therefore, an apparatus using a line memory instead of a frame memory is proposed in order to solve this problem. As for the apparatus conventionally proposed, reference is made only to correction processing in the case where an image is distorted in a trapezoid, and there is a problem that correction of other distortions, for example, barrel distortion or pincushion distortion is impossible, and the distortion remains in an output image.